The present invention concerns an oscillation rotation rate sensor that is excited in an x-y plane and comprises an external anchor means so that the degrees of freedom of the oscillation of a mass that is positioned relatively far outwardly is limited in a defined way. This provides advantages such as an increased shock robustness or also a reduced adhesion tendency relative to the substrate surface.
Among the multitude of concepts of micromechanical (MEMS) rotation rate sensor o structures, in particular the mechanically decoupled oscillation sensors are very effective and exhibit a high signal/noise ratio. They are based on the principle that the Coriolis force energy from one oscillation mode, the primary oscillation, is injected into the second oscillation mode, the secondary oscillation. The amplitude of the secondary oscillation is proportional to the rotation rate and can therefore be evaluated capacitively, for example.
Rotation rate sensors measure the rotation speed of a body. They are therefore used anywhere where the measurement of inertial movements such as acceleration and rotation rate are required:
automobile industrynavigation, driver assistance systems, ESP,rollover detectionaviation industrynavigationmilitary applicationsnavigationroboticsmeasuring; controllingbiomedicinesensing of motion sequenceslife sciencenavigation, sensing of motion sequences
Rotation rate sensors utilize as a measuring principally usually the effect of Sagnac interference (interference of light beams in a rolled glass fiber) or the Coriolis force in the form of a gyro or a moved spring-mass system. In particular the latter can be greatly miniaturized by manufacturing methods of micro-electro mechanical system technology (MEMS) and thus can be produced more efficiently and less costly.
In FIG. 3 the operating principle of rotary oscillation gyroscopes is illustrated. FIG. 3A shows schematically the function of a rotary oscillation gyroscope. A mass (sense+drive mass, i.e., an oscillating mass that, under the effect of the Coriolis force, moves simultaneously out of the oscillation plane) is attached by a spring to an anchoring point. This spring is construction-technologically designed such that its movement is reduced to 2 degrees of freedom:                a rotation movement about the Z axis (drive axis, drive mode) with spring constant kΦZ         a rotation movement about the Y axis (sense axis, sense mode) with spring constant kΦY         
The rotation rate about the X axis is measured.
The oscillation mass is caused to carry out a resonant movement so that an oscillating velocity vector or an oscillating moment of inertia is impressed. Accordingly, the sensor concepts are grouped into OMV (oscillating momentum vector) and OVV (oscillating velocity vector). A Coriolis force that is acting thereon causes a corresponding orthogonal deflection movement of the oscillating mass (coupled) and can thus be detected.
In a mechanically decoupled oscillation rotation rate sensor (FIG. 3B) a mass (drive mass) is excited in the primary oscillation with two degrees of freedom. This rotation rate sensor comprises accordingly a first mass arranged in an x-y plane (also referred to as primary oscillator, drive mass or drive element,) that is excited to perform an oscillating vibration and a second mass that is used for sensing (also referred to as secondary oscillator, sense mass or sense element) that is connected by connecting members (springs) with the first mass. The Coriolis force that is induced by an impressed rotation rate leads to a deflection in the second degree of freedom. Decoupling can be effected in different ways. In the first variant, the Coriolis force will act, when for example a rotary movement Ω of the sensor occurs about an axis perpendicular to the oscillation axis, on the mass points of the body that is undergoing rotary oscillation; by suitable measures this force is transmitted onto the sense mass. Its deflection in this dimension is then detected by suitable means, for example, capacitive electrodes. In this connection, coupling between the two masses or elements is ideally such that the rotary oscillation is not transmitted onto the second mass of the second element (sense mass). This can be realized in that this mass is suspended such that it has only a single degree of freedom of movement. In the second variant, the rotary movement of the first mass is transmitted onto the second mass so that the Coriolis force acts on the mass points of both bodies, wherein the first mass however is suspended such that it cannot be tilted or deflected out of the x-y plane.
The first variant has the advantage that the detection of deflection and thus of the rotation rate can be measured with the drive rotation having only little effect. Since the primary movement in drive mode is not detected or only partially detected, the useful signal has a higher signal-to-noise ratio. However, it is still disadvantageous in this connection that the drive element upon action of a rotation rate is tilted out of the x-y plane; this deteriorates the drive performance. In the second variant, these advantages and disadvantages are switched.
In all variants, the two oscillation modes can be in principle translation or rotation movements. The present invention concerns also rotation rate sensors whose active drive is realized by translation or rotation, with rotatory systems being preferred. Rotatory systems are in general less sensitive with regard to translatory shock injection and oscillation injection.
Since the principles for both systems are the same (see, for example, DE 196 41 284 C1), the basic principles and their embodiment upon which the present invention is based will be discussed in the following only with the aid of rotatory systems wherein however it is to be understood that translatory systems are encompassed also by the invention.
In case of rotation rate sensors and especially MEMS sensors there are a plurality of system approaches and, accordingly, there are many publications and patent applications. In the following table some important publications are listed that are based on the principle of the oscillation gyroscope. The columns are also indicate the input and output axes. The axis to be sensed is defined as the input axis. The drive axis results from the movement direction of the primary mode or drive mode which can be either a translatory (trans) or a rotatory (rot) periodic movement. The sense axis in general is positioned perpendicularly to the rotation axis and to the drive axis. The axis identifiers X, Y, and Z refer to a sensor of planar configuration wherein X and Y are positioned within the sensor plane and Z is perpendicular to this component plane.
TABLE 1inputaxisdrivesensepatenttype(sensing)axisaxisdecoupledU.S. Pat. No. 4,598,585OMVZYXnorotrotU.S. Pat. No. 5,203,208OMVZYXnorotrotU.S. Pat. No. 5,535,902OVVXZYnorotrotU.S. Pat. No. 5,650,568OVVXZYnorotrotU.S. Pat. No. 5,635,640OVVXZYnorotrotU.S. Pat. No. 6,505,511OVVZXYyestranstransU.S. Pat. No. 5,895,850OMVZXYyesXYZtranstransU.S. Pat. No. 6,062,082OVVXZXnorotrotEP0906557OVVXZYyesrotrotWO02/16871OVVZXYyestranstransU.S. Pat. No. 5,955,668OVVXZYyesrotrot
Two of the publications listed in the above table concern rotatory mechanically decoupled rotation rate sensors with oscillation of the excitation element in x-y plane and defection of the detection element about the y axis. In this connection, the two different variants that have already been discussed above in principle are realized. In EP 0906557 B1 a rotation rate sensor with decoupled orthogonal primary and secondary oscillations is disclosed. The primary oscillator is attached by means of a primary oscillator suspension centrally on the substrate and secures by means of torsion springs a secondary oscillator located in the same plane and provided as a sensing element, wherein the torsion springs transmit the induced oscillation of the primary oscillator rigidly onto the secondary oscillator. For one rotation of the sensor about a plane that is perpendicular to the plane in which the two oscillation elements are located, the Coriolis force acts on both elements. While the secondary oscillator is thereby caused to be tilted out of its plane, the primary oscillator remains in its plane because, on the one hand, it is anchored on the substrate in such a way that tilting out of this plane is not easily possible and, on the other hand, the torsion springs prevent a retransfer of the Coriolis force acting on the secondary oscillator onto the primary oscillator.
The proposal according to U.S. Pat. No. 5,955,668 is based on the reverse approach. The oscillation element that is excited to perform a radial oscillation is arranged in an annular shape about the tiltable sensor element that is attached by means of two anchors to the substrate. Flexible springs connect the oscillation element with the sensor element and are designed such that they neither transmit the oscillation of the oscillation element onto the sensor element nor retransmit the tilting movement of the sensor element caused by Coriolis force onto the oscillation element.
In FIG. 4, the principal configuration of a rotatory mechanically decoupled rotation rate sensor according to the second variant is illustrated schematically. A movable mass element (8) (sense mass) is suspended centrally on an anchoring point (12) by means of a spring structure (10). By means of a second spring structure (11) a further mass element (9) (drive mass) is connected fixedly to (8). The mass element (9) is excited to perform a periodic movement about the z axis.
The dimensions are constructively designed such that    1. the mass element 9 is substantially provided with two degrees of freedom of movement:            a rotatory movement about an axis that is parallel to the Z axis or even about the Z axis itself with a velocity vector vZ2         a rotatory movement about an axis that is parallel to the Y axis or about the Y axis itself with a velocity field vY2             2. the mass element 8 in the ideal case has only one degree of freedom, that is a rotational movement about the Y axis with a velocity field vY1.
When the velocity vector VZ1 of the mass 8 is equal to or almost zero, this is referred to as a mechanically decoupled system relative to this rotatory degree of freedom. A movement of the mass 9 about the Z axis does not transmit movement energy onto the mass 8 in this situation.
In operation, the mass element 9 is now caused to perform a periodic oscillation about a parallel line that is parallel to the Z axis at frequency fx and maximum velocity amplitude vZ2 (primary oscillation, primary mode). When the oscillation system is rotated about the sensitive axis X with a rotation rate ωx, onto the moved mass 9 a pseudo force, the Coriolis force, is acting that is defined by the following equation:{right arrow over (F)}Cor.=2*m2*({right arrow over (ω)}×{right arrow over (V)}Z2(t))with    FCor. Coriolis force    m2 mass of the primary oscillator 2    ωx externally impressed rotation rate    VZ2 speed or velocity of the primary mass.
Accordingly, an oscillating force field perpendicular to the X-Y plane is generated which causes a deflection of the mass (9) out of the plane. The Z deflection (secondary mode) is transmitted by means of spring element 11 onto the mass element 8. The magnitude of deflection of the mass element 9 can be used therefore directly for determining the rotation rate.
The excitation of a micro-electro mechanical structure can be effected, for example, in the following way: electrostatically, for example, by applying voltage onto finger electrodes, piezoelectrically or magnetically by injecting a magnetic field.
The measurement of a deflection can be realized in many ways, for example, capacitively by means of electrodes with reference space, difference-capacitively by means of paired spaced electrodes, electrostatically by means of electrodes with reference space, piezoelectrically, piezoresistively or optically.
Most rotation rate sensor systems are based on capacitive measurement of the deflection of the sense mass. The latter is in general embodied as a thin plate in the X-Y plane. In FIG. 6 it is schematically illustrated how the rotation rate sensors according to the invention, in section along the X-Z or the Y-Z axis, may be configured and how they can be manufactured. The illustrated configuration is particularly beneficial because it is comprised of only a few components and enables an integral configuration of anchor, oscillation element, connecting elements (springs) and detection elements: a substrate, for example, a silicon wafer, is covered with a structured sacrificial layer, for example, with an oxide that can be dissolved by a suitable etching agent or solvent. On top, a layer that can be structured and is made of a material such as polysilicon is applied from which the oscillation element, the anchoring structure, the connecting elements, and the sensing elements are to be formed (FIG. 6a). The layer that can be structured is connected directly to the substrate at the location of the future anchor. It can be structured exclusively two-dimensionally, for example, by suitable measures such as exposure to light through an aperture mask and subsequent dissolving of the unexposed and non-crosslinked surfaces (FIG. 6a). Subsequently, the sacrificial layer is dissolved and removed.
The capacitive measurement can be done most precisely when a differential capacity is measured, see FIG. 5 showing a substrate 13 with applied counter electrodes (measuring electrodes) A and B (14 and 15) as well as the plate 16 that upon rotational movement of the sensor about an axis X is tilted in Y direction (in this Figure the paper plane is the Z-Y plane).
In an ideal manufacturing process in the rest position the resulting differential capacity ΔC is zero. In reality, a pre-tilting of movable parts out of the horizontal rest position is often observed. This can be caused, for example, by anisotropic material properties. Examples for this are the presence of micro-crystallites in polysilicon layers, anisotropic layer stress, thermal layer stress or similar effects.
By means of the capacity difference in the useful signal an offset value is injected in the evaluation electronics. Since corresponding amplifier stages therefore must have a higher dynamic, a pre-tilting leads to reduced resolution of the sensor system. Current MEMS sensor systems can resolve for a band width of 25 Hz a rotation rate of 0.1°/s.
The rotation rate of 0.1°/s typically leads to an oscillation about the Y axis with the amplitude of 8×10−7 degrees for the mass 9 and of 4×10−7 degrees for the mass 9, see FIG. 4. This corresponds to a capacity change of 3 aF for the mass 8. The basic capacity is 3 pF; it is therefore necessary to resolve capacity changes in the ppm range.
On the other hand, either during manufacture or during the course of the service life of the sensor element a contact between movable sensor elements and/or stationary elements may occur. In an unfavorable case, when the restoring force is too small, by means of numerous adhesion forces the deflection is partially or permanently maintained so that the sensor is no longer functional and remains adhered (“sticking”). A mechanical shock or oscillation effect can also cause an intimate contact of movable sensor elements so that they jam or stick. In particular, the outer sensor structures that are far removed from an anchoring point have as a result of lever action a higher adhesion tendency. By increasing the restoring forces, for example, by springs of greater width, the movable structure can be constructed to be more stiff. However this causes also the useful signal to be reduced because the Coriolis force leads to a reduced plate deflection and thus to a reduced sensitivity.
It is an object of the present invention to provide rotation rate sensors of the aforementioned kind in which the sensitivity remains satisfactory but the robustness relative to parasitic environmental effects is improved in order to obtain a good balance between these parameters that affect one another mutually.